The continuing demand for energy has promoted the search for oil and gas at deeper horizons and in more hostile environments. Both circumstances require the use of more casing strings (steel pipes) in a given well than in past wells. Since the required size of the production casing and tubing string is relatively fixed, large diameter casing, set at increasing deeper depths, is invariably required in deep or hostile environment wells. Similarly, the concern for environmental pollution has promoted the development of relatively clean geothermal energy sources. Those wells also require the use of relatively large diameter casing strings. Furthermore, cleanup of existing pollution and increasing bans against surface dumping have fostered a new industry and technology relating to disposal and neutralizing wells. These wells also generally require large diameter casing set at relatively deep depths.
The performance of a pipe string for an oil, gas, geothermal or disposal well is, in part, measured by its ability to carry any one or any combination of the five primary forces which can be placed on a pipe string, namely 1) pressure, which may be internal and/or external, 2) axial force, which may be applied in tension and/or compression, 3) bending, 4) torsion and 5) lateral (side) loads. Thus, in designing a pipe string for a given application, the weight (wall thickness) and steel strength level (grade) of the pipe for the specific size (outside diameter) in question are selected with reference to the magnitudes of the foregoing forces that are expected to be exerted on the string.
The pipe string itself is composed of many sections of pipe fastened together at the well site by one of several available means. Some of these involve welding the pipe sectionals together as they are installed in the well. Others involve mechanical casing connections, including threaded or collet type connections. A key factor governing the success of large diameter casing designs is the ease and surety with which connections can be made between individual lengths in the field. Welding is slow and cumbersome. Consequently, for well casing service mechanical connectors are often employed. Due to the weight of the pipe itself, large diameter mechanical casing connections must be damage-resistant and possess good stabbing characteristics. This means that at the start of make-up they should be relatively insensitive to the relative positions of the members bearing the male and female connector parts, and that they should be capable of reliably achieving the required made up state. For proper make-up, collet connectors should achieve a positive lock-up, and threaded connectors should be quite resistant to cross-threading. Therefore, the performance of the connector is a dominant factor in the performance of the entire pipe string. A sound pipe string in the well cannot be achieved without sound connections.
The performance requirements of large diameter threaded connectors on large diameter casing strings are essentially the same as those for smaller size casing connections; yet it is far more difficult to satisfy these requirements in the larger sizes. The ideal tubular connector would be completely transparent in both load capacity and geometry, meaning that the pipe string would behave in all respects as if it were a single continuous length of pipe with no connections in it.
Welding comes close to achieving this goal. However, except for shallow strings set in high clearance holes and for drive pipe, welding the pipe sections together end-to-end is not practical since the actual welding and the inspection process is very time consuming and the pipe string would likely become stuck in the hole before the setting depth is reached. Some form of mechanical connector is therefore required.
Unfortunately, the ideal transparent mechanical connector has not been realized to date. Essentially transparent load capacity has been achieved with threads machined on hot forged upsets at the ends of the pipes. However, this has been accomplished at the expense of string geometry, i.e. the resultant connections are very bulky. Conversely, transparent geometry has been achieved with flush joints made within the confines of the pipe bodies, but only with substantial degradation of load capacity.
A good compromise between transparent geometry and transparent load capacity can be obtained with threaded and coupled (T&C) connections. In fact, this configuration is utilized by two of the three standard industry connections, American Petroleum Institute (API) 8R (LTC and STC) and BTC connections. Unfortunately, in such relatively large casing sizes as 16 to 20 inches in outside diameter, these API standard connections have serious deficiencies limiting their use to relative shallow depths and benign environments. The chief deficiencies with the 8R thread for large diameter pipe are extreme difficulty of achieving a proper makeup and very poor axial load capacity. Although API BTC is easier to make up than API 8R, it is subject to cross threading and lacks adequate pressure integrity.
To overcome the deficiencies of the industry standard API connections, various proprietary tubular connections have been developed that at least partially embody the above-described compromise, but are based on divergent design philosophies. For example, the proponents of one school of thought have been influenced by the perception that it is not generally practical to hot forge the ends of large diameter pipes and then machine integral connections on them. The pipe upsetters used to make such forgings on smaller pipes are usually restricted to outside pipe diameters of 103/4 inches or less. Therefore, this school has pursued proprietary connectors machined on separate forgings which are then welded to the pipe body. This resulted in massive connectors that were very robust. Both threaded and collet type connecting mechanisms of this type have been made available. However, a major problem with weld-on connectors is the difficulty of preventing misalignment between the pipe body and the forged, weld-on connector. Moreover, although near transparent load capacities can be achieved with this type of connection, it is only achievable at substantial expense to geometric transparency and at very high costs.
A different design school has concentrated on developing highly intricate and complex integral joint threads machined on plain-end pipe or plain-end pipe with cold formed ends. Although these proprietary thread forms have, or can achieve, near transparent geometry, their load capacity is deficient. Chiefly, the pressure and axial load capacity is substantially less than that of the pipe body. Also, virtually all of these connections have little if any bending resistance. Consequently, these joints are not suitable for use in directional (purposely deviated) wells, or where large diameter pipe strings must be set at deep depths.
Thus, it is believed that the proprietary connections developed according to these differing schools of thought do not provide an acceptable balance between load capacity, geometric transparency and ease of installation at a reasonable cost.
Another serious problem in the art relating to large diameter casing is that relatively few permutations, i.e. different weights and grades in a given size, exist in the industry standard API casing list. Because many well operators are required to use only API standard casing, well designers are often forced to choose between specific forms of casing that are either marginally adequate in load capacity or overly strong. Seldom is a specific large diameter casing product available that is exactly what is needed. For example, in the API large diameter casing sizes from 16 to 20 inches, there are only nineteen items with a unique combination of size (O.D.), weight (wall thickness), and grade (steel strength level) from which the well designer can choose. Moreover, even the strongest items do not have an adequate load capacity for the large diameter casing strings set at the deeper depths currently required in some wells. Therefore, there is a clear need to broaden the range of products available.
Some manufacturers have recognized the deficiencies of the standard API product and have developed remedies, even including additional available weights and grades in a threaded and coupled (T & C) configuration. However, it does not appear that any of them have developed a complete optimum solution. Because of specific limitations in their manufacturing processes, those "trans API" casing products now commercially available in permutations offering alternatives to API casing in terms of size, weight and grade, unfortunately fail to meet the rather crucial outside diameter and drift (internal free passage) diameter requirements of the API casing specifications. Furthermore, the dimensions of the raw coupling (female joining component prior to threading) does not conform with API requirements. These factors adversely affect performance and increase the cost of the product to the user.
Fortunately, in the diameter range of 16 to 20 inches, there are now about 300 different forms of API Line Pipe products. They include unique combinations of size, weight and grade, have specifications documented in the form of industry standards, and are thus potentially useful as large diameter casing. Moreover, the API Line Pipe specifications (SPEC 5L) extend to a grade 45 percent stronger than what is available in the API large diameter casing specification (Spec 5CT). Unfortunately, although API Spec 5L details a threaded line pipe connection in the sizes of 16 to 20 inches outside diameter, it is only standardized for the lightest weight pipe, which is not useful for casing service. Moreover, this connection is inherently unsuitable for downhole casing service.
There is therefore a need for a connector that can be applied to API line pipe or other tubular products which will allow it to properly function as large diameter casing. Such a connector should achieve a good balance between geometric and load capacity transparency, be easily installable in the field, and not possess any of the undesirable characteristics of the previously described prior products. Furthermore, this connector should be manufacturable with existing machinery at a realistic cost. The object of this invention is to provide a connector having some portion and preferably all of the foregoing advantages.